Method of coating wire screen cloth

ABSTRACT

Wire cloth screen is coated with an abrasive and corrosive resistant material by immersing the wire cloth into a chromic acid solution and then heating to form a hard chromic oxide coating. The coating process may be repeated to obtain the desired coating thickness. The coating serves to bond the wires together at the points of intersection and the heating stress relieves the wire.

United States Patent Jacobs May 20, 1975 [54] METHOD OF COATING WIRESCREEN 3,640,776 2/1972 Hanes 148/6.16 CLOTH 3,687,738 8/1972 Malkin148/62 3,717,509 2/1973 Palm et al..... l48/6.l6 [75] Inventor: LouisJohn Jacobs, Chicago, I11. I [73] Assignee: Combustion Engineering,Inc., Primary Examinerl-eon Rosdol Wi C Assistant Examiner-Edith L.Rollins Attorney, Agent, or FirmRichard H. Berneike [22] Flledz Dec. 21,1972 [21] Appl. No.: 317,335 [57] ABSTRACT Wire cloth screen is coatedwith an abrasive and cor- 521 US. Cl. l48/6.2; 117/99; 148/315 rosiveresistant material y immersing the wire cloth [51] Int. Cl. C23c 1/10into a Chromic acid Solution and then heating to form [58] Field ofSearch 148/6.l6, 31.5, 6.2; a hard chromic Oxide, coating- The coatingProcess 7 99; 1 2/34 199 may be repeated to obtain the desired coatingthickness. The coating serves to bond the wires together at 5 ReferencesCited the points of intersection and the heating stress re- UNITEDSTATES PATENTS 12/1951 Cohen 204/42 lieves the wire.

4 Claims, No Drawings 1 METHOD OF COATING WIRE SCREEN CLOTH BACKGROUNDOF THE INVENTION Wire screen cloth is used for many purposes and isoften subjected to abrasion, impact and corrosive, conditions. Forexample, wire screen cloth is used classifying equipment to separatematerials according to particle size and abrasion can be a seriou'sproblem. The screens are usually vibrated or moved in some manner toproduce relative motion between the screen and, the particles. Thematerial moving on the *screen abrades the screen and the most seriouswear occurs at,the intersections of the wires. At these intersectionsfine particles become lodged between the two wires and the abrasiveaction of the particles causes rapid destruction of the wire. i

During the formation of wire screen cloth, the wires are straightened,crimp'ed, or formed into special shapes and woven into the desiredpattern on the weaving machine. This processing of the wire work hardensthe metal. The work hardened wire resists abrasive action but it is alsosubject to more rapid failure by fatigue. It would therefore bedesirable to anneal the wire to increase its elastic limit. 7

SUMMARY OF THE INVENTION An object of the present invention is to treatwire' screen cloth so as to provide a coating thereon which will alsobond the wires together at the intersection. Another object of theinvention is to bond the wires together' to prevent the abrasive actionat the point of contact of the wires and to increase the load bearingproperties. A further object-of the invention is to provide such acoating on the wire screen cloth which will be abrasive and corrosiveresistant. These and other objects of the present invention are obtainedby treating the wire screen cloth to form a hard chromic oxide coatingthereon which also bonds together the wires at the intersections. Theformation of the chromic oxide coating involves a heating process whichhas the additional advantage of annealing the wire screen cloth.

DESCRIPTION OF THE PREFERRED EMBODIMENT Wire screen cloth to be treatedin accordance with the present invention is first cleaned to remove anygrease or oil from the surface. Also, if the wire is excessivelyoxidized, the oxide is removed such as by an acid treatment and thenslightly re-oxidized by rapidly heating and then cooling. Thisre-oxidation step forms a porous oxidizing layer. The wire is thenimmersed in a chromic acid solution which penetrates the oxide layer onthe wire surface. The wire screen cloth is then removed from thesolution and heated to convert the chromic acid to chromicoxideaccording to the following reaction:

4Cr0 2Cr O 30 This heat treating process produces a strong oxide bondbetween the porous oxidized layer and the base metal of the wire. Theimmersion-heating cycle is repeated as many times as desired to producethe desired coating thickness and to densify the porous oxide surface.Since chromic oxide is a very hard material having a hardness of 9.3 onthe Moh scale, this densified coating on the metal makes it resistant toabrasion.

Examples of three different types of chromic acid solu tionsfor coatingthe wirescreen cloth are as follows:

.SOLUTION A Chromium trioxideTechnical grade i Water to produce asolution with a specific gravity of Solution B is prepared by dissolvingthe chromium trioxide in'the water, bringing'the solution to a slowboil, and very slowly adding the'activated charcoal while stirring.Although the exact reaction which takes place is: not known, it isbelieved that the heating of the solutionwwi th the activated charcoalremoves some of theoxygenzwhich leaves the solution ascai'bon dioxidewith the Cro radicals linking to forrnlachain. The compounds which-areformed are thought to contain CrO radicals and hexavalent chromium whichare re..- ferred to as chromyl compounds, Slow boilingis con} tinueduntil no noticeable chemical reactiorris taking place. The resultingsolution has an increased viscosity,

SOLUTION C Typical Range 2 Chromium Trioxide, Technical Grade 100.00lbs. Zinc Oxide, Technical Grade 16.3 lbs. lO-2O lbs. Water to produce asolution with a specific gravity of 1.7

Then add a mixture of:

Typical Range Alumina (Alcoa T-61 minus 325 mesh) 61 lbs. 40-80 lbs.Silica (minus mesh) 39 lbs. 20-60 lbs.

The effect of the addition of zinc oxide is to increase the viscosity ofthe solution so that thicker coatings are applied in each treatmentstep. The alumina and silica are also added to produce a suitableconsistency for coating the wire. The amount of each is balanced toproduce a coating in which the thermal expansion and elasticity of thecoating is very similar to that of the base material.

It has been found that the wires can initially be coated with eitherSolution A or Solution B. However, the preferred procedure is to firstcoat with Solution A and then to increase the thickness of the coatingby using Solution B until the desired thickness has been obtained. Forscreens coarser than 6 mesh, the coating thickness can be increased morerapidly by using Solution C, preferably after three initial coatingswith Solutions A and/or B. Solution C would not normally be suitable forfine mesh screens since it builds up a thick coating rapidly and wouldgreatly reduce the opening size of the smaller screen. Although aspecific gravity of 1.7 has been referred to in the above examples,these specific gravities can be adjusted anywhere from 1.66

to 1.75'. However the specific gravity of 1.70 is the most ideal valuefor impregnation.

The wire Screen cloth with the coating of chromic acid thereon is thenheated to about 650 to l,200F such as by induction or radiant heat whichconverts the chromic acid to chromic oxide. The specific curingtemperature would depend upon the base metal of the screen and theeffect of the temperature on the heat treatment of that metal. The wirescreen cloth is then cooled to 350F or below and immersed in the desiredchromic acid solution again. The wire screen cloth is then heated toform another layer of chromic oxide. Since the chromic acid solutionreadily wets the wire screen cloth and penetrates the porous oxide layeron the wire, a strong bond is formed between the chromic oxide, theoxide layer on the wire and the base metal.

The chromic oxide layer which is formed on the wire is very dense,smooth and abrasion resistant. Theoxide coating bonds the wires togetherwhich strengthens and reinforces the wire screen cloth therebyincreasing the load bearing capacity. The heating of the wire screencloth to convert the chromic acid to chromic oxide also relieves thestresses induced into the wire structure during drawing, straightening,crimping and weaving of the wire cloth. This greatly increases thefatigue resistance of the wire. For example, it has been found that awire screen cloth treated in accordance with the present invention andinstalled on electrically vibrating screening machine will last 25percent longer than the same untreated wire screen cloth. This test wasrun with a fixed load on the wire screen cloth so that only the fatigueresistant factor could be evaluated.

While preferred embodiments of the present invention have beendescribed, it will be understood that these are merely illustrative andthat changes may be made without departing from the invention asclaimed.

What is claimed is:

l. A method of treating wire screen cloth so as to form a coatingthereon containing chromic oxide and to bond the wires together at thepoints of intersection comprising the steps of:

a. providing a wire screen cloth, the wires of said wire screen clothhaving a thin porous oxide layer thereon;

b. coating said wire screen cloth with a chromic acid solution; and

c. heating said wire screen cloth to a temperature sufficient to convertsaid chromic acid solution to chromic oxide and to heat treat said wirewhereby a coating of chromic oxide is formed on said wires and saidwires at the points of intersection are bonded together with chromicoxide.

2. A method of treating wire screen cloth as recited in claim 1 andfurther including the steps of repeating said coating step (b) and saidheating step (c) at least one additional time to form additionalcoatings of chromic oxide thereon.

3. A method as recited in claim 1 wherein said chromic acid solution hasa specific gravity of from 1.66 to 1.75.

in claim 1 wherein said chromic acid solution is a solution prepared bythe process of disolving chromium trioxide in water, slowly boiling saidsolution, adding activated charcoal to said solution in the ratio of lto 20 pounds activated charcoal to 266 pounds chromium trioxide, andcontinuing said slow boiling until no noticeable reaction is takingplace.

4. A method of treating wire screen cloth as recited

1. A METHOD OF TREATING WIRE SCREEN CLOTH SO AS TO FORM A COATINGTHEREON CONTAINING CHROMIC OXIDE AND TO BOND THE WIRES TOGETHER ATPOINTS OF INTERSECTION COMPRISING THE STEPS OF: A. PROVIDING A WIRESCREEN CLOTH, THE WIRES OF SAID WIRE SCREEN CLOTH HAVING A THIN POROUSOXIDE LAYER THEREON; B. COATING SAID WIRE SCREEN CLOTH WITH A CHROMICACID SOLUTION; AND C. HEATING SAID WIRE SCREEN CLOTH TO A TEMPERATURESUFFICIENT TO CONVERT SAID CHROMIC ACID SOLUTION TO CHROMIC OXIDE ANDTOHEAT TREAT SAID WIRE WHEREBY A COATING OF CHROMIC OXIDE IS FORMED ONSAID WIRES AND SAID WIRES AT THE POINTS OF INTERSECTION ARE BONDEDTOGETHER WITH CHROMIC OXIDE.
 2. A method of treating wire screen clothas recited in claim 1 and further including the steps of repeating saidcoating step (b) and said heating step (c) at least one additional timeto form additional coatings of chromic oxide thereon.
 3. A method asrecited in claim 1 wherein said chromic acid solution has a specificgravity of from 1.66 to 1.75.
 4. A method of treating wire screen clothas recited in claim 1 wherein said chromic acid solution is a solutionprepared by the process of disolving chromium trioxide in water, slowlyboiling said solution, adding activated charcoal to said solution in theratio of 1 to 20 pounds activated charcoal to 266 pounds chromiumtrioxide, and continuing said slow boiling until no noticeable reactionis taking place.